4GB-4GB Memory Split: Physical Address Extension (PAE)

Although Red Hat speaks of the 4GB-4GB Memory Split in its discussion of the 2.6 kernel, this terminology has nothing to do with the new kernel. The memory split is a Red Hat enhancement to the Enterprise and Fedora kernels; it is not integrated into the main source tree (it is not part of the 2.6 kernel).

The 32-bit CPUs are limited in that they can address only 2³² bytes (4 gigabytes) of memory. With the Pentium Pro, Intel introduced a work-around to this limitation called Physical Address Extension (PAE), which permits the operating system to address up to 64 gigabytes of memory. Because they are limited to addressing 4 gigabytes each, 32-bit programs cannot access this much memory. A Linux kernel from the main tree is able to allocate up to 1 gigabyte for the kernel and 3 gigabytes for each userspace (page 1062) process.

The kernels shipped with Red Hat Linux are patched to allow the kernel to allocate up to 4 gigabytes for itself and 3.7 gigabytes for each userspace process. These limitations affect Linux on 32-bit architectures only. A 64-bit Linux kernel on a 64-bit CPU, such as a SPARC64, UltraSparc, Alpha, or Opteron, is able to access up to 16 exabytes (16 x 2⁶⁰ bytes) of RAM.